Method of making tempered shaped part

ABSTRACT

A method of making a tempered part has the steps of first heating a metal blank to above a first predetermined temperature and then preshaping the heated blank while still above the first temperature. The preshaped blank is then reheated to a second tempering temperature, and the reheated blank is given its final shape while still above the second temperature in a press. The blank with its final shape is cooled below the second tempering temperature while still in the press.

FIELD OF THE INVENTION

The present invention relates to the manufacture of tempered parts. More particularly this invention concerns a multistep heat-tempering method.

BACKGROUND OF THE INVENTION

A tempered part is typically made by first heating a blank, then preshaping it, and finally shaping, tempering, and cooling or quenching it in a press.

In vehicle manufacture, more and more vehicle parts of strong and high-strength steel are used to do reduce vehicle weight. This is particularly true for chassis elements, for example the structure and/or safety elements such as the door crash barriers, A and B columns, shock absorbers or longitudinal and horizontal beams, which are manufactured increasingly by heat-shaping and tempering in a press to achieve tempered steel with tensile strengths greater than 1,000 MPa to meet weight targets and safety requirements.

GB 1,490,535 describes a process for press shaping and tempering of a steel sheet with low material thickness and good dimensional stability is known in which a boron-alloy steel plate is heated to a temperature above the AC₃ point and subsequently in fewer than five seconds is pressed into its final shape between two indirectly cooled tools with a significant shape change and then, while remaining in the press, is subjected to a quick cooling or quenching so that the desired martensitic and/or bainitic structure is achieved. As a result of these steps, one obtains a product with high accuracy of shape, good dimensional accuracy and high strength that is exceptionally well suited for structural and safety parts in vehicle manufacture. In the following, this is the process that is meant by heat-shaping and press tempering. This way, preshaped parts as well as flat metal blanks can be heat shaped and press tempered. For preshaped parts, the shaping stage can also be a shaping of a few percent of the final geometry or be limited to a calibration.

U.S. Pat. No. 7,004,004 describes a process for manufacturing a tempered part from tin sheet. Here, metal blank is heated and then preshaped, given its final shape, and tempered in one heat. This way, the heat loss of the metal blank is minimized by suitable tools and/or optimized handling times. The process is particularly suited for processing a metal blank that is coated with aluminum or an aluminum alloy that is unwound from a coil. The heat loss of the metal blank can be minimized by short pressure contact times in the preshaping stage, which is particularly suited to simple shapes that can be made in only two steps.

US 2007/0163683 describes a process for manufacturing a part by shaping a coated metal blank of heat-treated steel where prior to shaping the metal blank is placed in a first furnace in a first step and is austenitized there. The time the metal blank remains in the first furnace is selected in such a way that in addition to the desired microstructural transformation, the thickness of the layer increases. Then, the metal blank is quickly cooled and the heat-treated metal blank is subsequently stored for a while. Then the metal blank is briefly reheated in a second furnace up to austenizing temperature immediately prior to shaping into the part and, after the microstructural transformation has taken place, the blank is shaped and tempered. In this way, the processes are decoupled. For the actual shaping of the metal blank, little space is required and even less infrastructure. Intermediate storage of the heat-treated metal blank is possible, so that heat treatment for, among other things, influencing the AlSi layer can easily also be done by the steel manufacturer or sheet-metal supplier.

US 2006/0137779 describes a process for manufacturing a metallic shaped part from a semifinished product of untempered, steel plate that can be heat shaped. The semifinished product is shaped in a cold-shaping stage, particularly by deep-drawing, into and is subsequently trimmed at the edges so that it has an edge contour closely resembling the part that is to be manufactured. Finally, the trimmed part blank is heated and press tempered in a heat-shaping tool. The part created thereby already has the desired edge contour after heat-shaping, so that a final trimming of the edge of the part is eliminated. In this way, the cycle times of manufacturing tempered parts from steel plate can be reduced considerably.

OBJECTS OF THE INVENTION

It is therefore an object of the present invention to provide an improved method of making a shaped tempered part.

Another object is the provision of such an improved method of making a shaped tempered part that overcomes the above-given disadvantages, in particular that produces a geometrically demanding part of good quality.

SUMMARY OF THE INVENTION

A method of making a tempered part has according to the invention the steps of first heating a metal blank to above a first predetermined temperature and then preshaping the heated blank while still above the first temperature. The preshaped blank is then reheated to a second tempering temperature, and the reheated blank is given its final shape while still above the second temperature in a press. The blank with its final shape is cooled below the second tempering temperature while still in the press.

The invention is based on the recognition that thin sheet metal only has a small capacity to hold heat much of which is already regularly dissipated in large part into the shaping tool in a single shaping step. Even in a preshaping process with a fast press or a two-step tool, the capacity to store heat is sufficient only for limited shaping. Complex parts must be shaped in several steps. Up to now, preshaping of uncoated heat-treated steel, for example, of a 22MnB5 quality, is done as a rule with cold shaping steps. However, the resistance to deformation is significantly less in heated, heat-treated steel. Consequently, heated steel can be deep drawn more than it can be cold shaped. To this end, heating to below the AC₃ point is sufficient. Heating to the AC₃ point only needs to be done for tempering. By heating prior to shaping, in the case of complex parts, several cold-shaping steps can be replaced with a single heat-shaping step, meaning of shaping at increased temperature. For this reason, the invention is advantageous for complex parts. A demanding geometry is, for example, a deep profile and difficult edges.

Heat-treated steel that is coated with aluminum or with aluminum alloy should only be heat shaped. The aluminum coating guarantees protection against decarbonization and scaling during the entire heating, heat-shaping and the heat-treatment process and subsequently protects the completed part from corrosion. However, in the conventional hot-dip coating processes, an inter-metallic phase forms in the coating process between the base material and the coating, that on the one hand ensures that the coating does not melt off in the heat-shaping process, on the other hand, but on the other hand cannot be sufficiently cold shaped. The relatively refractory intermetallic phase can also tear or chip off during cold shaping, with the consequence that protection against corrosion in the completed final part is no longer ensured. Complex shapes can only be manufactured in accordance with the invention by preshaping the coated metal blank warm and by heating it again for the final shaping up to at least the tempering temperature. However, with each heat, the aluminum coating layer experiences an increase in layer thickness, depending on the duration of heating. Care must be taken here that the coating does not exceed a layer thickness that is critical for subsequent welding.

BRIEF DESCRIPTION OF THE DRAWING

The above and other objects, features, and advantages will become more readily apparent from the following description, reference being made to the accompanying drawing in which:

FIG. 1 is a flow chart illustrating the instant invention;

FIG. 2 a detail view of a variant on the FIG. 1 method; and

FIG. 3 is a perspective view of a longitudinal beam for a vehicle chassis made by the method of this invention.

SPECIFIC DESCRIPTION

As seen in FIG. 1 a metal blank is cut in a cutting station 2 from a supply coil 1. This metal blank is heated in a first furnace 3 and subsequently preshaped in a first heat-shaping station 4 and then cooled. If needed, the now preshaped part is trimmed in another station 5. For uncoated heat-treated steel it is advantageous when the first heating in the first furnace 3 is to a temperature below the AC₃ point because then there is no or only very little tempering in the preshaping station 4 because of the absence of austenizing. Moreover, the costs of heating are lower. An untempered part can also be trimmed more easily.

Subsequently, the preshaped and trimmed part is reheated in a second furnace 30. As this is the last heating prior to final shaping in connection with setting the degree of hardness, the preshaped part is heated to a temperature above the AC₃ point of the alloy for the tempering step at which it is maintained for austenizing. Then, in a second, force-cooled heat-shaping station 40, the final shaping and tempering takes place. Depending on the tolerances and geometry, after tempering another trimming must also be done in a cutting station 50.

In a special embodiment of FIG. 2, the invention can also be applied to a so-called tailor-rolled blank. To this end, steel strip is unrolled from the supply coil 1 and partially reduced in thickness by rolling in a roll stand 5. Subsequently, the partially rolled strip is straightened in a straightening station 6 and then fed into station 2 for cutting the metal blank. The completed part thus is adapted to a made-to-measure thickness of defined section.

FIG. 3 shows a longitudinal beam 14 for a vehicle chassis manufactured in a two-step heat-shaping process according to the invention. The longitudinal beam 14 is closed on three sides 18, 19, 20 over its entire longitudinal extension. In addition, it transitions from its rear end region 15 through a distinctly bent center region 16 into a front end region 17. Starting with a flat metal blank, the profile that is closed on three sides 18, 19, 20, cannot be simultaneously shaped and bent. A one-time heating prior to the first shaping step is not sufficient for two shaping steps. The longitudinal beam 14 would cool too much after the first shaping step and before the second shaping step.

The longitudinal beam manufactured in accordance with the invention is made of 22 MnB5 heat-treated steel coated with aluminum. This type of coated steel cannot be cold shaped without damaging the coating. Alternatively, this longitudinal beam 14 could be manufactured only as a multi-part profile from two half shells and be welded together, in which case the two half shells would be separately heat shaped and subsequently overlapped in the middle and welded. But a multipart longitudinal beam with a welding seam does not provide the kind of performance during an accident that is provided by the one-piece longitudinal beam 14 manufactured in accordance with the invention and also requires more material and is heavier 

1. A method of making a tempered part, the method comprising the steps of sequentially: heating a metal blank to above a first predetermined temperature; preshaping the heated blank while still above the first temperature; reheating the preshaped blank to a second tempering temperature; final shaping the reheated blank while still above the second temperature in a press; and cooling the blank below the second tempering temperature while still in the press.
 2. The method defined in claim 1 wherein the blank is cooled in the press by contact with cooled tools of the press.
 3. The method defined in claim 2 wherein the first temperature is below the second temperature.
 4. The method defined in claim 1 wherein the preshaping is done in several steps.
 5. The method defined in claim 1, further comprising the step after preshaping and before reheating of: trimming the workpiece.
 6. The method defined in claim 1, further comprising the step before heating to the first temperature of: providing the metal blank with an aluminum coating.
 7. The method defined in claim 6, further comprising the step after final shaping of using the finally shaped part as a longitudinal vehicle-chassis beam, the blank being made of 22MnB5 steel and having a generally U-shaped profile and a nonstraight longitudinal extension. 